1. Field of the Invention
The present invention relates to a frequency modulator and to a transmitting/receiving device and an integrated circuit having such a device.
2. Description of the Background Art
The invention resides in the field of discrete frequency modulators for transmitting/receiving devices in communications systems. Such frequency modulators generate an output signal with an instantaneous frequency that is a function of an n-level modulation signal, wherein the modulation signal can assume N≧2 different discrete modulation values, and a predetermined frequency value of the output signal is associated with each modulation value.
The invention resides more particularly in the field of so-called direct modulating frequency modulators, which generate the output signal as part of frequency synthesis. It resides, furthermore, more particularly in the field of so-called “closed loop” frequency modulators, whose phase locked loop (PLL) is closed even during modulation.
A direct modulating frequency modulator with a closed phase locked loop is known from the article entitled, “Automatic Calibration of Modulated Frequency Synthesizers,” by Daniel R. McMahill and Charles G. Sodini published in the technical journal, “IEEE Transactions on Circuits and Systems—II: Analog and Digital Signal Processing,” Vol. 49, No. 5, May 2002, pages 301-311. In this connection, the PLL synthesizer is modulated in that the divisor of the frequency divider in the feedback path of the phase locked loop is varied as a function of the modulation signal.
The modulation bandwidth, which is to say the maximum possible rate of the modulation signal, is limited by the bandwidth of the closed phase locked loop with such an approach, however. Larger modulation bandwidths are made possible by providing a digital preemphasis filter whose transfer function must precisely match the inverse transfer function of the PLL (see section II.C and, in particular, FIG. 3 of the article). In order to achieve this matching, the cited article proposes an automatic calibration unit that controls the current in the charge pump as a function of the output signal of the voltage controlled oscillator (VCO) and of the modulation signal (see section III and, in particular, FIG. 4). A disadvantage here is the complexity of the calibration algorithm and the associated high complexity of implementing the calibration unit (see section IV and, in particular, FIGS. 7, 14, 17, 18 and 20). Furthermore, the high consumption of operating energy associated with the high implementation complexity is disadvantageous. Moreover, the calibration unit itself requires a disadvantageous dead time before establishment of calibration as a result of time delay elements (phase accumulators, filters, integrators, etc.).
Another direct modulating frequency modulator with a closed phase locked loop is known from the article, “A 43 mW Bluetooth Transceiver with −91 dBm Sensitivity” by Christian Cojocaru et al., which appeared in the conference proceedings of the “2003 IEEE International Solid-State Circuits Conference,” on pages 64, 65 and 432. Here, too, the PLL synthesizer is modulated in that the divisor of the frequency divider in the feedback path is varied as a function of the modulation signal. Moreover, the modulation signal is filtered and is fed to a second control input of the oscillator through a digital-to-analog converter. A disadvantage here is the increased implementation complexity, particularly due to the filter and the two analog-to-digital converters, as well as the increased energy consumption associated therewith. Moreover, the frequency change per unit time of the output signal is relatively small.